Somatostatin receptors in the Rhesus monkey brain: localization and pharmacological characterization

To characterize the nature and distribution of somatostatin (SRIF) receptors, radioligand binding studies and in vitro receptor autoradiography were performed in Rhesus monkey brain using either ¹²⁵I]LTT-SRIF-28 (Leu⁸,D-Trp²²,¹²⁵I-Tyr²⁵]SRIF-28) alone or in the presence of 3 nM seglitide (to block sst₂ sites), ¹²⁵1]Tyr³-octreotide or ¹²⁵I] CGP 23996 (cAsu-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Tyr-Thr-Ser]) in buffer containing either 120 mM Na⁺ or 5 mM Mg²⁺. ¹²⁵I]Tyr³-octreotide labelled an apparently homogeneous population of sites in cerebral and cerebellar cortex (B _max = 27.3±2.8 fmol/mg protein and 52.6±8.6 fmol/mg protein, pK_d = 9.46±0.03 and 9.93±0.03, respectively). The pharmacological profile of these sites correlated highly significantly with that of human recombinant sst₂ receptors (r = 0.996), but not or much less with that of human recombinant sst₃ and sst₅ receptors (r = 0.12 and 0.45, respectively). ¹²⁵I]CGP 23996 (in Na⁺-buffer) also labelled an apparently homogeneous population of sites in Rhesus monkey cerebral cortex membranes (B _max = 3.1±0.3 fmol/mg protein, pKd = 10.57±0.08), the pharmacological profile of which was highly significantly correlated with the profiles of human recombinant sst₁ and sst₄ receptors (r = 0.98 and 0.96, respectively).Using receptor autoradiography, high levels of ¹²⁵I]LTT-SRIF-28 and ¹²⁵I]Tyr³-octreotide recognition sites were found in basal ganglia, molecular and granular layers of the cerebellum and layers III, V and VI of entorhinal cortex. In these regions, the addition of 3 nM seglitide produced a marked decrease of ¹²⁵I]LTT-SRIF-28 binding. Low levels of ¹²⁵I]LTT-SRIF-28 binding were observed in subiculum, pituitary and choroid plexus. By contrast, ¹²⁵I]CGP 23996 labelling in the presence of Mg²⁺ as well as Na⁺ ions was highest in pituitary and choroid plexus. However, ¹²⁵I]CGP 23996 binding was diversely affected by these ionic conditions in several regions of hippocampus and cerebral cortex. Displacement of ¹²⁵I]CGP 23996 (in Mg²⁺-buffer) with seglitide in the molecular layer of the cerebellum, deep layers of the entorhinal cortex, layers I, II and V of the insular cortex and frontal pole yielded complex competition curves suggesting the presence of two populations of SRIF receptors. By contrast, ¹²⁵I]CGP 23996 binding (in Mg²⁺-buffer) in the choroid plexus, hilus of the dentate gyrus and stratum oriens and radiatum of the CA3 field of hippocampus was not affected by seglitide up to 10 M, suggesting only sst₁ and/or sst₄ sites which have a negligible affinity for seglitide to be present in these structures.Taken together, these results suggest that ¹²⁵I]CGP 23996 (in the presence of Na⁺) labels exclusively SRIF-2 receptors (sst₁ and/or sst₄), whereas in the presence of Mg²⁺ ions, ¹²⁵I]CGP 23996 labels both SRIF-2 and SRIF-1 receptors (sst₂, sst₃ and sst₅). The present study also demonstrates the presence and differential distribution of sst₂ and sst₁/sst₄ receptors in the Rhesus monkey brain.